OOOI times are very important to the air transport industry for a variety of reasons. Most airlines pay their flight crews in dollars per hour based on recorded OOOI times. The FAA evaluates an air carrier's on time performance by comparing recorded OOOI times to published flight schedules. The air carrier's dispatch operations use the actual "Off" time during flight to forecast arrival times, make gate assignments, and plan ground support. A late arriving flight can necessitate a new gate assignment, which impacts ground operations from baggage handling to refueling.
Currently, some airlines require their flight crews to record OOOI times by hand on a per flight basis. The flight crew then verbally relays the "Out" and "Off" Times after departure to their ground based dispatch operations via their VHF Transceiver. Upon arrival, the flight crew hand delivers the written "On" and "In" Times to the gate agent, who subsequently enters the times at a computer terminal. The Captain's Clock in the flight deck is used as the time source. Other airlines use an Aircraft Communications Addressing and Reporting System (ACARS) Management Unit (MU) and VHF Transceiver to automatically forward OOOI times to their ground based dispatch operations. The ACARS approach has some drawbacks. Passenger and cargo doors are typically wired with sensors that detect door closure. The parking brake also has to be wired with a sensor to detect when it is released or set. These discrete wires have to be installed and routed to the ACARS MU, along with the landing gear squat switch discrete, to provide it the information it requires to define the OOOI times. The cost of using the ARINC provided ACARS service to forward OOOI times is expensive by commercial wireless telecommunication standards. The cost associated with ACARS is what motivates some airlines to favor the manual approach.
The "Out" time is defined as the moment in time when the aircraft pushes back from the gate. ACARS defines this as the moment when all passenger and cargo doors are closed and the parking brake is released. Brake pressure drops from greater than 3,000 psi to less than 100 psi. Prior to pushback, power to the aircraft transitions from ground power to internally provided Auxiliary Power Unit (APU) power. Power from the APU is required to start the first engine. If the APU is working, the engines are started after pushback. A "tug" is typically used to push the aircraft backwards, away from the Jetway. In some situations, aircraft push back under engine power. The tug turns the aircraft so that once the engines are started, the aircraft is pointing in a direction where it can proceed forward under engine power. This event s marked by a change in magnetic heading from the constant heading while at the gate. If the APU isn't working, then the number one or left engine is started at the gate, before disconnecting from ground power. The right engine, on the same side as the cargo doors, usually remains off until after pushback, to accommodate any last minute baggage loading requirements.
Many airlines do not keep their parking brakes engaged the entire time the aircraft is parked at the gate. Because the brakes get very hot during landing and taxi in, they are prone to binding or warping as they cool down if left engaged for long periods of time. Therefore many carriers choose to set the parking brake once "In" at the gate only long enough to allow the ground crew to position the chocks on either side of the wheel(s) to prevent the aircraft from moving. Once the chocks are secured, the parking brake is released. The parking brake is set again as part of the flight deck preparation process, prior to pushback. The parking brake is engaged before the chocks are removed. Once removed, the parking brake remains engaged until pushback. The release of parking brake and associated change in brake pressure defines the "Out" time. Engaging the parking brake at the gate and the associated change in brake pressure defines the "In" time. Other parameters are used to segregate the "Out" and "In" events from routine braking during taxi operations.
Once the engines start, the aircraft proceeds with the taxi operation until the aircraft receives Air Traffic Control clearance to take off. The Air/Ground Relay is monitored to detect the precise moment when the aircraft wheels leave the runway. This time is recorded as the "Off" Time, i.e., weight off wheels. When the aircraft reaches its destination, the time when the aircraft wheels come into contact with the runway is recorded as the "On" Time, i.e., weight on wheels. The aircraft then taxis to its assigned gate. The moment that the parking brake is set is recorded as the "In" Time.
It would be desirable to eliminate the need for flight crews to manually record OOOI times. Because many of these times are recorded during high workload phases of flight, removing this requirement from flight crews improves flight safety. The main benefit of this method is improved accuracy and reduced operating costs. The current process is subject to human error and "gaming", particularly if the flight crew's pay check directly benefits if the flight duration is artificially inflated. Conversely, an air carrier might avoid a penalty for poor on time performance by artificially shortening flight duration. Although the people involved in the process possess high levels of personal integrity, it is nevertheless advantageous to eliminate the temptation with an automated system. From an operating cost perspective, a significant amount of labor is eliminated with an automated process. Currently the flight crew phones the recorded "Out" and "Off" times back to dispatch after takeoff and hands the recorded "On" and "In" times to the gate agent once back on the ground. The gate agent then forwards this information to a data entry person, where it is hand input into a payroll database where it affects payroll actions and an on time performance analysis database.
Harris Corporation of Melbourne, Fla. manufactures a product referred to as Ground Data Link (GDL), which provides a wireless system of transferring data files to and from air transport aircraft while on the ground at GDL equipped airports. The basics of the system are disclosed in a co-pending patent application entitled, "Wireless, Frequency-Agile Spread Spectrum Ground Link-Based Aircraft Data Communication System", filed Nov. 14, 1995, U.S. Ser. No. 08/557,269, now U.S. Pat. No. 6,047,165 the disclosure which is hereby incorporated by reference in its entirety.
Miller, Jr. et al describe a system for similarly recording and processing telemetry data. In the Miller art, recorded information is periodically retrieved manually via a Ground Readout Unit, which connects to their Airborne Integrated Data System via a conventional wired data port. Miller also provides for an interface to an Aircraft Communications Addressing and Reporting System (ACARS), which is a low data rate (2.4 kbps), high usage cost, air to ground transceiver used, from a practical standpoint; to send high priority "snapshots" of recorded to data to ground stations while airborne. In Miller, parametric data is gathered to assist in both aircraft maintenance and to monitor aircraft and crew performance, but does not use OOOI times, or any means of deriving them from parametric data.